The present invention relates to a transfer device which is provided with a cleaning unit having a magnetic brush roller and which functions to electrostatically transfer a toner image on an image carrier to a transfer material held on a dielectric body.
Transfer devices such as electrophotographic copying machines are generally provided with cleaning units for removing the toner particles that remain on an image carrier comprising, for example, a photosensitive body, after transfer operation. One of the methods of removing remaining toner particles is a magnetic brush cleaning method in which a magnetic brush is formed by carrier particles magnetized by magnetic field formed by a magnet contained within a sleeve so that the toner particles are removed and recovered by the rotating magnetic brush. In this case, the carrier particles are formed by a material which is frictionally electrified to have the polarity reverse to that of the toner, and a bias voltage with the polarity reverse to that of the toner is applied to the sleeve.
For some reason, however, the remaining toner particles contain toner particles which are electrified to have the polarity reverse to the initial polarity. Such toner particles having the reverse polarity cannot be easily removed to an adequate extent by the magnetic brush cleaning method. In order to increase the cleaning performance, the polarity of the toner particles on a photosensitive body is made uniform with the initial polarity of the toner before cleaning by a discharger. If discharge is performed by applying a high potential for the purpose of making the polarity uniform, however, the photosensitive body and the toner particles become strongly electrified, leading to a deterioration in the cleaning performance. The voltage applied by the discharger before cleaning cannot be high and thus the toner particles having the reverse polarity cannot be easily removed to an adequate extent.
It has therefore been thought that a rubber blade could be pressed against the surface of the photosensitive body so that the reversely electrified toner particles are completely removed by mechanical force. This blade cleaning, however, has the disadvantage that damage of the surface of the photosensitive body may readily occur, resulting in deterioration of the quality of an image. The positional relationship between the photosensitive body and the blade is also limited. If an inappropriate positional relationship is established, the toner particles removed by the blade cannot be easily recovered, with the result that certain restrictions on layout are imposed. For example, if the blade is disposed just above the photosensitive body for cleaning off the toner particles, the toner particles cannot be easily recovered.